Network Working Group C. Perkins
Request for Comments: 2004 IBM
Category: Standards Track October 1996
Minimal Encapsulation within IP
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies a method by which an IP datagram may be
encapsulated (carried as payload) within an IP datagram, with less
overhead than "conventional" IP encapsulation that adds a second IP
header to each encapsulated datagram. Encapsulation is suggested as
a means to alter the normal IP routing for datagrams, by delivering
them to an intermediate destination that would otherwise not be
selected by the (network part of the) IP Destination Address field in
the original IP header. Encapsulation may be serve a variety of
purposes, such as delivery of a datagram to a mobile node using
Mobile IP.
1. Introduction
This document specifies a method by which an IP datagram may be
encapsulated (carried as payload) within an IP datagram, with less
overhead than "conventional" IP encapsulation [4] that adds a second
IP header to each encapsulated datagram. Encapsulation is suggested
as a means to alter the normal IP routing for datagrams, by
delivering them to an intermediate destination that would otherwise
not be selected by the (network part of the) IP Destination Address
field in the original IP header. The process of encapsulation and
decapsulation of a datagram is frequently referred to as "tunneling"
the datagram, and the encapsulator and decapsulator are then
considered to be the the "endpoints" of the tunnel; the encapsulator
node is refered to as the "entry point" of the tunnel, and the
decapsulator node is refered to as the "exit point" of the tunnel.
Perkins Standards Track [Page 1]RFC 2004 Minimal Encapsulation for IP October 1996
2. Motivation
The Mobile IP working group has specified the use of encapsulation as
a way to deliver packets from a mobile node's "home network" to an
agent that can deliver datagrams locally by conventional means to the
mobile node at its current location away from home [5]. The use of
encapsulation may also be indicated whenever the source (or an
intermediate router) of an IP datagram must influence the route by
which a datagram is to be delivered to its ultimate destination.
Other possible applications of encapsulation include multicasting,
preferential billing, choice of routes with selected security
attributes, and general policy routing.
See [4] for a discussion concerning the advantages of encapsulation
versus use of the IP loose source routing option. Using IP headers
to encapsulate IP datagrams requires the unnecessary duplication of
several fields within the inner IP header; it is possible to save
some additional space by specifying a new encapsulation mechanism
that eliminates the duplication. The scheme outlined here comes from
the Mobile IP Working Group (in earlier Internet Drafts), and is
similar to that which had been defined in [2].
3. Minimal Encapsulation
A minimal forwarding header is defined for datagrams which are not
fragmented prior to encapsulation. Use of this encapsulating method
is optional. Minimal encapsulation MUST NOT be used when an original
datagram is already fragmented, since there is no room in the minimal
forwarding header to store fragmentation information. To encapsulate
an IP datagram using minimal encapsulation, the minimal forwarding
header is inserted into the datagram, as follows:
+---------------------------+ +---------------------------+
| | | |
| IP Header | | Modified IP Header |
| | | |
+---------------------------+ ====> +---------------------------+
| | | Minimal Forwarding Header |
| | +---------------------------+
| IP Payload | | |
| | | |
| | | IP Payload |
+---------------------------+ | |
| |
+---------------------------+